ROCK CRYSTAL, the colorless variety of quartz SPECIMENS Rock Crystal is the name given to all clear colorless quartz. It is widely used as a popular ornamental stone and is also used as a gemstone . Although it is one of the least expensive gemstones, cut rock crystal has been used as imitation diamonds . Rock crystal lacks the fire, color (of course) and the rarity to be ranked as a fine precious gemstone. None-the-less, rock crystal is in wide use as a gemstone due to its beauty, affordability, availability, and ease of cutting. The most common use for rock crystal is in ornamental carvings. A well known ornamental carving for rock crystal is the crystal ball of fortune telling fame. While rock crystal is common it is hard to find large crystals of quartz with the clarity and size required for the crystal balls that most of us associate with fortune tellers. Rock crystal is used for many ornamental carvings from spheres (crystal balls) to pyramids to obelisks to figurines to eggs to bowls to wands. There are also many fine chandeliers that are outfitted with rock crystal ornaments.

Rock crystal can have the colorless clarity of pure water, unlike ordinary window glass which is a pale green. Yet the most common flaws - internal fractures - result in veils and colorful refractions that lend beauty to an otherwise ordinary object. Rock crystal is an alternate and traditional birthstone for the month of April. Quartz is often associated with balance, clarity, and energy. Many people prefer uncut clusters of rock crystal. These natural treasures look like crystal cities of fantasy stories. The long slender clear prisms of quartz projecting upward from a common base are true mineralogical wonders that can be admired by all. They cost of good clusters of rock crystal is quite affordable and comes from sources around the world. The best rock crystal sources are in the famous Hot Springs area of Arkansas, USA; Cumberland, England; St. Gotthard, Switzerland; Brazil and Madagascar. Large individual crystals of quartz have been found in Brazil, the largest weighs over 44 tons. Rock crystal often has inclusions of other minerals and these inclusions sometimes produce popular varieties of ornamental stone. Golden rutile inclusions produce a unique stone aptly named rutilated quartz that has a very hair-like look.

Another apply named stone is called tourmalinated quartz and contains intricately crossing needles of black schorl tourmaline trapped in the clear crystal. Phantoms are a result of inclusions which occur when other minerals such as chlorite , hematite or even milky quartz form as a crust on the surface of the crystals during a hiatus in the crystal's growth. The crystal then grows out and around the encrusting growth, encasing it inside. But since the thin encrustations formed over a crystal complete with crystal faces it appears that there is a crystal inside the crystal. The encrustations are often incomplete or diffuse and appear ghostly, hence the name phantom. Rock crystal is only one of several quartz varieties. Other varieties that form macroscopic (large enough to see) crystals are as follows: Amethyst is the purple gemstone variety. Citrine is a yellow to orange gemstone variety that is rare in nature but is often created by heating Amethyst. Milky Quartz is the cloudy white variety. Prasiolite is the leek-green variety. Rose quartz is a pink to reddish pink variety. Smoky quartz is the brown to gray variety.

Amethyst Galleries' QUARTZ (Silicon Dioxide) Smoky Quartz Quartz is the most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz. A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad categories. The specimens could be separated by answers to the following questions: color ?, shade?, pyramidal ?, prismatic ?, druzy ?, twinned ?, sceptered ?, phantomed ?, included ?, tapered?, coated?, microcrystalline?, stalactitic ?, concretionary ?, geoidal ?, banded?, etc. Multiple combinations of these could produce hundreds of unique possibilities. Some macrocrystalline (large crystal) varieties are well known and popular as ornamental stone and as gemstones. Amethyst is the purple gemstone variety. Citrine is a yellow to orange gemstone variety that is rare in nature but is often created by heating Amethyst.

Milky Quartz is the cloudy white variety. Prasiolite is a leek-green gemstone variety that is rare in nature but is created by heating Amethyst from certain locations. Rock crystal is the clear variety that is also used as a gemstone. Rose quartz is a pink to reddish pink variety. Smoky quartz is the brown to gray variety. Cryptocrystalline (crystals too small to be seen even by a microscope) varieties are also used as semi-precious stones and for ornamental purposes. These varieties are divided more by character than by color. The primary varieties of chalcedony are as follows: Agate is a banded variety (sometimes with translucent bands) Bloodstone is green with red speckles Carnelian is yellow to orange Check out our selections of agate bookends , available as natural agate , dyed blue , and dyed purple . Quartz is not the only mineral composed of SiO2. There are no less than eight other known structures that are composed of SiO2. These other substances and quartz are polymorphs of silicon dioxide and belong to an informal group called the Quartz Group or Silica Group.

All members of this group, except quartz, are uncommon to extemely rare on the surface of the earth and are stable only under high temperatures and high pressures or both. These minerals have their own unique structures although they share the same chemistry, hence the term polymorph, which means many forms. Quartz has a unique structure. Actually, there is another mineral that shares quartz's structure, and it is not even a silicate. It is a rare phosphate named berlinite , AlPO4, that is isostructural with quartz. The structure of quartz involves corkscrewing (helix) chains of silicon tetrahedrons. The corkscrew takes four tetrahedrons in order to repeat itself, or three turns. Each tetrahedron is essentially rotated 120 degrees. The chains are aligned along the C axis of the crystal and interconnected to two other chains at each tetrahedron making quartz a true tectosilicate . This structure is not like the structure of the chain silicates or inosilicates whose silicate tetrahedronal chains are not directly connected to each other. The structure of quartz helps explain many of its physical attributes.

For one, the helix makes three turns and this helps produce the trigonal symmetry of quartz. Likewise a helix or corkscrew lacks mirror planes of symmetry as does quartz. The corkscrew structure would also disrupt any cleavage which requires a plane of weakness not found in quartz and breakage would result in the curved fracture, conchoidal, that is found in quartz. Quartz can also have left and right handed crystals just as a corkscrew can screw in a left handed way or in a right handed way. There are even some very difficult to identify crystals of quartz that are twinned with alternating one sixths of the crystal being right handed and then left handed. Quartz is a fun mineral to collect. Its abundance on the Earth's surface is incredible and produces some wonderful varieties that don't even look like the same mineral. A collector must always be up on the many varieties of quartz and it sometimes embarrasses a collector to have collected too many specimens of such a common mineral. But nearly all collectors concede that you can never really have enough quartz specimens. Note that quartz is nearly a defining component of most geodes .

The agate form typically lines the original cavity, creating a durable shell to contain subsequent mineral growth, which itself is often quartz crystals of one or more varieties. Geodes from Brazil are a good example, as layers of agate comprise the shell, lined with milky and/or colorless quartz crystals, topped with amethyst. PHYSICAL CHARACTERISTICS: Color is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous. Transparency: Crystals are transparent to translucent, cryptocrystalline forms can be translucent or opaque. Crystal System is trigonal; 3 2. Crystal Habits are again widely variable but the most common habit is hexagonal prisms terminated with a six sided pyramid (actually two rhombohedrons ). Three of the six sides of the pyramid may dominate causing the pyramid to be or look three sided.

Left and right handed crystals are possible and identifiable only if minor trigonal pyramidal faces are present. Druse forms (crystal lined rock with just the pyramids showing) are also common. Massive forms can be just about any type but common forms include botryoidal, globular, stalactitic, crusts of agate such as lining the interior of a geode and many many more. Cleavage is very weak in three directions (rhombohedral). Fracture is conchoidal. Rock and Mineral Samples Rock and Mineral Samples Amethyst Amethyst has been used to make jewelry and valued as a gemstone since the time of the ancient Egyptians. Currently In Stock More Info Amethyst Point Unlike the amethyst cluster, an amethyst point is a single amethyst crystal. Amethysts are popular collectible minerals and have been valued as gemstones since ancient times. Currently In Stock More Info Aventurine Aventurine is a green quartz with crystals too small to see with the naked eye. Aventurine is found in shades of silver, yellow, red, brown, and green.

Currently In Stock More Info Black Onyx Black Onyx is cryptocrystalline, which means that your sample is composed of crystals so tiny that they cannot be seen by the naked eye. Currently In Stock More Info Black Tourmaline Tourmaline can come in a variety of colors, but black is very common. It can be found in igneous and metamorphic rocks. Currently In Stock More Info Blue Kyanite Blue Kyanite forms under tremendous pressure. How much pressure? Imagine a schoolbus balanced on one wheel on top of a quarter. Now imagine another on top of the first. That is about 50-60 pounds per square inch of pressure, which are the conditions inside of the earth where kyanite is formed. Currently In Stock More Info Blue Quartz The blue color in blue quartz is due to the scattering of light from tiny inclusions inside the crystal matrix. Currently In Stock More Info Chalcedony Chalcedony is a term that applies to a wide variety of quartz with cryptocrystalline crystal structure. Agate, aventurine, onyx and chrysoprase are all examples of chalcedony.

Currently Not In Stock More Info Chrysocolla Chrysocolla is a lightweight mineral with a bright blue-green color. Although is has a bright color, chrysocolla is brittle and rarely used for jewelry. It's streak can range from white to blue-green. Currently In Stock More Info Chrysoprase The apple-green color in chrysoprase comes from tiny inclusions of nickel inside of the cryptocrystalline (tiny crystal) matrix. The color of chrysoprase makes it favorite of rock and mineral collectors. Currently Not In Stock More Info Citrine Point Citrine is a tan-yellow variety of quartz, one of the most abundant minerals in the Earth's crust. Currently In Stock More Info Emerald Quartz Currently In Stock More Info Fluorite Fluorite is a defining mineral on Moh's Scale of Hardness (it's #4). When you cleave fluorite, the resulting surface is so perfectly smooth that it shines. Currently In Stock More Info Frozen Quartz Quartz is one of the most abundant minerals in the Earth's crust. Your clear quartz sample is sometimes known as "frozen quartz", "pure quartz" or "rock crystal". Currently In Stock More Info Garnet Garnets have been used by humans since the bronze.

Species of garnet can differ chemically, but they all share similar physical properties such as color, hardness, and streak. Currently Not In Stock More Info Geodes Geodes (also known as thunder eggs) are spherical rocks that are partially filled with minerals. These geodes are filled with quartz, which is the most abundant mineral found inside of geodes. The physical properties listed below are for the quartz interior and not the rock exterior. Currently In Stock More Info Gold Tigers Eye Gold Tigers Eye is an example of a chatoyant mineral due to the fibrous nature of the matrix. Currently In Stock More Info Green Calcite Calcite is a very common mineral and comes in a variety of different colors. It is a defining standard on Moh's scale of hardness (it's number 3). Currently In Stock More Info Green Tigers Eye Green Tigers Eye is an example of a chatoyant rock (oiel de chat is french for "cat's eye"). Chatoyance refers to the fibrous structure of the mineral's matrix. Currently In Stock More Info Gypsum Rosettes Also known as the "desert rose", gypsum rosettes are formed in arid, sandy conditions.

Gypsum is one of the defining standards (gypsum is #2) on Moh's scale of hardness. Currently In Stock More Info Hematite Hematite is a favorite of collectors. It has cool nicknames like red ochre and Alaska black diamond, it is found on Mars, it streaks red, it has a metallic luster, and is attracted to magnets. This is a fun mineral to add to your collection! Currently In Stock More Info Howlite Howlite is a mineral that is commonly used in art and jewelry making. One interesting property of howlite is that it is porous and will readily accept dyes. Currently In Stock More Info Lepidolite Mica Currently Not In Stock More Info Orange Calcite Calcite is a very common mineral that comes in a wide variety of colors. Calcite is also a standard (it is number 3) on Moh's scale of hardness. Currently In Stock More Info Peacock Pyrite Peacock Pyrite is another name for chalcopyrite, an important mineral ore. When exposed to oxygen in the air, chalcopyrite turns into a variety of bright colors, thus the name "peacock".

Currently Not In Stock More Info Petrified Wood Petrified Wood is actually a fossil created by an ancient tree. Over many years, the wood was replaced by minerals (in our case, quartz). Currently In Stock More Info Pumice Pumice is an igneous rock that is created when super-heated rock is ejected from a volcano. As the rock cools, bubble form which give the pumice a "sponge-like" appearance. Currently Not In Stock More Info Pyrite Pyrite is also known as fools gold because of its metallic luster and gold coloring. It is the most common of the sulfide minerals. Currently In Stock More Info Red Tiger Eye Some collector's call it "Tiger's Eye" and some call it "Tiger Eye"... both are commonly used. Notable sources of tiger's eye are the United States, Canada, Korea, South Africa, Burma, India, Brazil, China, and Spain. Currently In Stock More Info Rose Quartz The pink color in rose quartz is due to trace metals such as titanium, iron, or manganese. Rose quartz is always found in a solid mass and rarely forms crystals. Why? Scientists aren't sure.... Maybe you can solve this mystery.

Currently In Stock More Info Rough Amazonite Amazonite is also known as the amazon stone. The name originates from stones of similar color that were discovered in the Amazon Basin. Division of Geology and Mineral Resources - Minerals DIVISION OF GEOLOGY AND MINERAL RESOURCES Rocks, Minerals, and Fossils Common Minerals of Virginia A wide variety of minerals occur in Virginia. These minerals form the basic building blocks of the rocks that shape the landscape. Minerals exhibit distinctive properties that makes each unique. In Virginia minerals are mined for industrial purposes, collected by enthusiasts, and used for scientific research to help us better understand the Earth. The list of minerals below highlights a few of the most common or popular minerals that are found in Virginia. If you would like more details on the distribution of minerals in our state, click here . HClick on each image below for a more detailed view. Amazonite KAlSi3O8 A variety of microcline feldspar that typically forms in coarse-grained igneous rocks called pegmatites. The presence of lead results in a greenish or blue-green color. Amazonite is occasionally used as a gemstone. This mineral is mined in Virginia near the town of Amelia.

Epidote (Ca,Ce,Pb,Sr,Y)2(Al,Fe,Mn,V)3Si3O12(OH) or (Ca,Ce,Pb,Sr,Y)2(Al,Fe,Mn,V)3Si3O11(OH,F)2 A mineral that typically appears as a pistachio green granular mass within a low-grade metamorphic rocks, such as greenstone. Kyanite Al2SiO5 Forming aggregates of long and thin bladed crystals, this pale blue mineral is commonly found in metamorphic rocks such as gneiss or schist. Kyanite is actively mined in Virginia in Buckingham County. Click here for more information. Photo courtesy of Rob Lavinsky. Staurolite (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2 Staurolite forms interesting brownish black twinned crystals, also known as ”Fairy Stones” that can appear in medium-grade metamorphic rocks such as gneiss or schist. Staurolite can be found in Patrick County, Virginia. Photo courtesy of Rob Lavinsky.

Hematite Fe2O3 An iron oxide mineral, hematite is commonly metallic gray, brown, or reddish in color. It is an ore of iron and can form naturally in sedimentary, igneous, or metamorphic rocks. Hematite is a primary constituent in what is perhaps the most famous rock in the world – the Banded Iron Formation which formed as Earth’s atmosphere first became oxygenated 2.2 billion years ago. Although the Banded Iron Formation is not found in Virginia, hematite has historically been mined in the Valley and Ridge Province. Feldspar (Na,K)AlSi3O8 Feldspar is an extremely common mineral. It can be found in many varieties of rock, especially metamorphic or igneous rock types. Feldspar is usually milky white, but can have color due to impurities in the crystal structure. Feldspar is currently mined in central Virginia and is very common in the Piedmont and Blue Ridge Provinces. Photo courtesy of Rob Lavinsky. Mica KAl3Si3O10(OH)2 or K(Mg,Fe)3AlSi3O10(OH)2 This distinctive mineral is easy to identify by its platy appearance.

Commonly seen as shiny flecks in sedimentary, igneous, or metamorphic rocks, mica easily flakes off into thin crystal plates. Two common varieties of mica are muscovite (typically pale in color) and biotite (darker brown to black). Mica is commonly found in the Piedmont and Blue Ridge Provinces of Virginia. Photo courtesy of Rob Lavinsky. Calcite CaCo3 As a very common mineral, calcite can be seen in many different rocks including limestone, dolomite, and marble. Although calcite is commonly white, it can appear in various colors due to chemical impurities. Calcite can be distinguished from other similar looking minerals (such as quartz and feldspar) by its effervescent reaction with weak hydrochloric acid. Calcite can be found throughout Virginia. Photo courtesy of Rob Lavinsky. Quartz SiO2 One of the most common minerals in the world, quartz can be found in sedimentary, metamorphic, and igneous rocks throughout Virginia. Quartz typically appears as clear or whitish crystals within a rock such as granite. Amethyst, agate, chalcedony, chert (flint), and opal are all varieties of quartz.

Click here for more information on quartz veins in Virginia. Click here for more information on blue quartz in Virginia. Pyrite FeS2 Often mistaken for gold, pyrite is easily recognizable by its metallic luster and brass-yellow color. Pyrite forms into cubic crystals or as a mass of shapeless grains within sedimentary nodules, coal seams, or other rock veins. It is commonly associated with gold and found in the Gold-Pyrite belt of Virginia. Photograph courtesy of Rob Lavinsky. Click here for more information. Garnet (Ca, Fe, Mg, Mn)3(Al, Cr, Fe, Mn, Si, Ti, V, Zr)2(SiO4)3 Typically dark reddish brown, garnets are commonly found in medium- to high-grade metamorphic rocks such as schist of the Piedmont and Blue Ridge Provinces. Gold Au Probably the most popular mineral in Virginia is gold. Gold occurs in small amounts throughout a large portion of the Piedmont and Blue Ridge Provinces. Click here for more information about gold. DMME Headquarters Quartz: The mineral Quartz information and pictures Advertising Information The Mineral quartz Quartz is one of the most well-known minerals on earth.

It occurs in basically all mineral environment s, and is the important constituent of many rock s. Quartz is also the most varied of all minerals, occurring in all different forms, habits, and colors. There are more variety names given to Quartz than any other mineral. Although the Feldspar s as a group are more prevalent than Quartz, as an individual mineral Quartz is the most common mineral. Most mineral reference guides list Chalcedony as an individual mineral, but in reality it is a variety of Quartz. It is the microcrystalline form of Quartz, forming only occurs in microscopic, compacted crystals. This page deals only with the crystalline forms of Quartz. Chalcedony is listed on its own dedicated page in this guide. Other important varieties of Quartz, such as Amethyst , Citrine , and Agate , also have dedicated pages due to their popularity and varieties. Some forms of Quartz, especially the gemstone forms, have their color enhanced. Almost all forms of the yellow-brown variety Citrine are in fact heat treated . Much Amethyst is also heat treated to intensify color, and a green transparent form known as "Green Amethyst" or "Prasiolite" is formed by heat treating certain types of Amethyst.

There is also a transparent sky blue form of Quartz crystals, as well as a wildly iridescent type that are synthetically colored by irradiation of gold. In some localities, Hematite forms a thin red or brown layer internally in the Quartz crystal, giving it a natural bright red to brown coloring, and sometimes even a mild natural iridescence . Quartz frequently forms the inner lining of geode s. Most geodes have an inner layer of larger crystalline Quartz, and an outer layer of Chalcedony or banded Agate . For additional information, see the gemstone section on Quartz . Crystal Forms and Aggregates Crystals, which are hexagonal in shape, vary in shape and size. Quartz crystals are unique and very identifiable with their pointed and often uneven termination s. Crystals can be in enormous prismatic and stubby crystals, or in pointed aggregate s of such crystals. Crystals are usually striated horizontally, and are sometimes doubly terminated .

Quartz crystal habits include drusy , grainy , bladed , as linings of geode s, as rounded waterworn pebbles, radiating , as pointy pyramids on a matrix , as dense agglomerations of small crystals, massive , globular , stalactitic , crusty , in nodules , and in amygdule s. Crystals frequently twin ; a famous twinning habit is the Japanese twin , where two crystals contact at a 90º angle. Quartz crystals may also contain a scepter growth, where the top of a crystal bulges out from the rest of the crystal, and may also form as phantom growth , where one crystal forms over another, leaving a ghosted form inside. The crystal structure of Quartz is a very complicated. As a result of a changeover from alpha to beta Quartz, crystals form as hexagonal prisms with modified crystal face s. Click here for a detailed explanation on the crystal structure of Quartz. USES Quartz is an important mineral with numerous uses. Sand, which is composed of tiny Quartz pebbles, is the primary ingredient for the manufacture of glass.

Transparent Rock Crystal has many electronic uses; it is used as oscillators in radios, watches, and pressure gauges, and in the study of optics. Quartz is also used as an abrasive for sandblasting, grinding glass, and cutting soft stones. It is also essential in the computer industry, as the important silicon semiconductors are made from Quartz. In addition to all the practical uses, Quartz is essential to the gem trade. (See the separate Quartz gemstone page.) Many varieties are faceted as gem s. Amethyst and Citrine are the most well-known gem varieties. Rose Quartz , Smoky Quartz , Rock Crystal , and Aventurine are also cut or polished into gems. Small colorless Quartz crystals are worn by some as pendants for good luck. Quartz is also a very popular among collectors. Certain collectors specialize their collection entirely on Quartz alone. NOTEWORTHY LOCALITIES Excellent Quartz specimens come from numerous localities on all the corners of the globe. Only a few select localities are mentioned here. Some of the largest crystals, weighing many tons and yet still perfectly formed, come from several pegmatite mines in the state of Minas Gerais, Brazil.

A very well known French locality of excellent Rock Crystal clusters is Bourg d'Oisans in Isere, especially at the La Gardette Mine. In the U.S., flawless Rock Crystal of exceptional quality and abundance comes from Arkansas, most notably in the Hot Springs area, Garland Co.; Mount Ida in the Ouachita Mountains, Montgomery Co.; and the Jeffrey Quarry, Pulaski Co. Very large crystals also come from the pegmatite mines in the San Diego Co., California. Lustrous, doubly terminated stubby crystals come from Middleville and Little Falls, Herkimer Co.; and St. Johnsville, Montgomery Co., New York, where they are known affectionately as Herkimer Diamond s. Although Rose Quartz is very common, good crystals are rare and only found in a handful of localities. These are mostly in Minas Gerais, Brazil, in Galileia (especially at Lavra da Pitorra); and in Itinga, in the Jequitinhonha Valley; and also at Newry, Oxford Co., Maine. Fine Smoky Quartz comes from the Pikes Peak area, El Paso Co., Colorado. The Alps contain two very classic occurrences in Switzerland in St.

Gotthard, Uri, Switzerland; and in Chamonix-Mont Blanc, France. The finest Rutilated Quartz comes from Novo Horizonte and Ibitiara, Bahia, Brazil. Tall prismatic Quartz colored dark green from Hedenbergite inclusion s come from Serifos Island, Greece. Exceptional Faden Quartz comes from the Dara Ismael Khan District, Waziristan, Pakistan. common minerals common minerals Non-metallic Minerals and their Properties N.B. most (if not all) minerals with colour listed as 'colourless' may be tinted almost any colour by the presence of trace impurities. NAME in veins, as stalactites, stalagmites lime fertilizer, cement, 1 perfect, parallel to crystal faces evaporites colourless in rock crystal - see guide to properties for other varieties white gemstone.

concrete and glass making (as sand) 'cockscomb' structures in crystalline form vitreous 3 good, 1 horizontal (basal), 2 vertical (at 90 degrees to basal) heaviest mud in the oil industry Biotite mica silvery-white (named after Moscow in 'White Russia' white vitreous, sometimes a very slightly greasy or watery appearance 2.7 colourless, blue & yellow as blue john variety white 4 4 perfect octohedral:- cleavage planes cut across corners of 6-sided cubic crystals to leave 8-sided octohedral cores. blue john as blue john (from French; bleu-blue jaune-yellow). Also used as flux colourless (may be cloudy), white, pink or pale red, other pale colours also white 6 (a moh's scale mineral) 2 cleavages at 90 degrees sub- and as an abraisive in scouring powders. Plagioclase white or grey to grey-blue or other pale colours white 2 cleavages at almost 90 degrees sub- is named after its 2 cleavages at almost and as an abraisive in scouring powders.

Oligoclase 2 cleavages at almost 90 degrees 6 - 6.5 and as an abraisive in scouring powders. Olivine 2.5 - 3.5 (more often ~3.5) green, may also be yellow or brown white 1 poor cleavage, cracks on what appears to be second cleavage plane; actually a sub-parallel fracture 6 - 7 deep red, crimson, purple, brown, black, olive, greens, pink, yellow white 7 - 7.5 (except gossular variety, which may be as low as 6.5) a dense emerald green, or other shades of green light green, Metallic Minerals and their Properties NAME vein mineral, often with calcite, fluorspar and barites cheif source of lead (Pb). Haematite 2 poor cleavages sub-conchoidal or uneven fracture often in limestones as a relacement minerals, also in metamorfphic deposites, ironstones and as both thin veins and cement in sandstones eg.

the New Red Sandstone iron ore and used as a pigment in paint 'Red Ochre' Pyrite FeS2 cubic crystals, or as pentagonal dodecahedral crystals. Also as nodules with an internal structure of radiating needles, also as crystalline masses metallic cubic crystals often have striated faces, conchoidal fracture, sparks when struck with geological hammer pyrite = fire mineral. smells sulphurous when rubbed on a streak plate occurs as free crystals or nodules in coal, clay and shales, also in veins formerly source of sulphur, used to make sulphuric acid (native sulphur now main source). Sphalerite (aka zinc blende, black jack) ZnS usually massive aggregates, also tetrahedral crystals resinous, sometimes brilliant or adamantine on fresh surfaces 4.0 University of Minnesota’s Mineral Pages: Quartz ^ Top Did you know... Quartz was the mineral upon which the Stone Ages were based. With few exceptions, most early stone tools were fashioned of quartz. Outcrops of quartz that were suitable for tool manufacturing were targeted by some of the earliest known mining activities and the mined quartz was traded across vast distances, even before humans began to establish agricultural societies.

Even in our modern world, quartz is one of the most widely used minerals, though few people are aware of its many contributions. One of its most common uses is also its most transparent, as quartz is the source of most of our society’s glass- from windowpanes and crystal goblets to eyeglasses and cathedrals’ stained glass windows. Quartz is one of the most common minerals because it is chemically and physically stable at Earth’s surface conditions. It is a significant component of hydrothermal veins and felsic igneous rocks, and is often the dominant mineral in sandstones and siltstones, as well as their metamorphosed equivalents. Quartz occurs in a wide range of varieties under a bewildering array of informal names, such as jasper, flint, tiger’s eye, amethyst, citrine, chalcedony, onyx, opal and agate. Because of their abundance, the quartz varieties are often subdivided into informal groups. One of the most common subdivisions is ‘chert’, a term collectively used for all the quartz varieties that have crystals too small to be seen without a microscope. In this use, ‘chert’ is simply a subset of ‘quartz’.

Description and Identifying Characteristics Despite its variable appearance, Quartz’ hardness (greater than that of glass or most metals) and lack of cleavage make it relatively easy to distinguish from other minerals. Ancient Greeks called quartz ‘krystallos’, which means ‘ice’, a reflection of the clarity of quartz’ clear transparent crystals. Although most quartz crystals are clear and colorless, slight chemical impurities create a number of color varieties, some of which are common enough to have their own names. Translucent purple crystals are called amethyst which has been designated as February’s birthstone. Translucent pink to reddish quartz is known as rose quartz, while translucent gray and cloudy white crystals are respectively called smoky quartz and milky quartz. In a similar manner, quartz that is composed of microscopic crystals (chert) can occur in a wide variety of colors. The most common chert color varieties are flint (dark gray to black), jasper (red to red-brown), and onyx (mixed white and black). Agate is another variety of chert that exhibits distinct bands of color that formed as the chemistry of the fluids moving through its host rock changed slightly.

^ Top In Our Earth: The Geologic Importance of Quartz Quartz is a common component of silica-rich igneous rocks, forming up to 25% of the volume of granites. It also occurs in hydrothermal veins and pegmatites. Because quartz is relatively stable at surface temperatures and pressures, it can be concentrated by weathering processes to be even more abundant in sedimentary rocks than it is in igneous rocks. Quartz sand grains are a significant component of most sandstones and siltstones, and some detrital sedimentary rocks are almost exclusively composed of quartz grains. Metamorphism of these sedimentary rocks produces quartzite, a metamorphic rock composed almost entirely of quartz. Quartz’ stability, even under harsh chemical conditions, was aptly demonstrated by some dinosaurs that swallowed stones to aid in the digestion of land plants. These gizzard stones, called gastroliths, are almost always composed solely of quartz. Other stones the dinosaur may have swallowed broke down in the swirl of its strong stomach acids, only leaving quartz rocks to survive as working gastroliths. Although it is chemically resistant, quartz is weakly soluble so most natural waters contain some dissolved silica that can precipitate as quartz.

Hence quartz is a common vein-filling and cavity-filling mineral in rocks through which silica-rich waters have moved. Some microscopic marine plankton, like diatoms and radiolarians, take advantage of the dissolved silica in seawater to create shells of quartz. These can accumulate on the sea floor, particularly in deep sea areas. Under very high pressure, water can dissolve calcite and aragonite shells rather easily, while less soluble quartz shells are left to form quartz-rich deposits. Microscopic quartz crystals often form chert seams and nodules as silica-rich groundwater moves through a variety of sedimentary and metamorphic rocks. Chert precipitated from silica-rich groundwater can also replace bone and wood present in sediment and sedimentary rocks, often even preserving the materials’ original cellular structure. This process is the origin of ‘petrified’ wood. Since quartz occurs in so many different geological settings, it can be associated with many different minerals. In igneous rocks and pegmatites, quartz usually occurs with potassium feldspars, muscovite, biotite and amphiboles. In sedimentary rocks or metamorphosed sedimentary rocks, quartz may be associated with potassium feldspar, calcite, dolomite or a variety of clay minerals.

^ Top In Our Society: The Economic Importance of Quartz So many varieties of quartz have played such crucial roles in human society that untangling the web of names and terms is daunting. The word ‘quartz’ appears to have originated from the German or Saxon languages, but the meaning of the term is unknown. In our earliest human experience, the microcrystalline varieties of quartz played an unparalleled role in setting our ancestors on the path towards modern civilization. The many varieties of chert were some of the first materials beyond wood and bone to be sculpted into tools. When a piece of chert is struck, it can break with conchoidal fracture to create a sharp edge. Early humans learned to work chert, breaking it systematically to create tools. Now millions of years old, some of these tools still have remarkably sharp edges, a testimony to quartz’ stability. Later, humans learned to work obsidian (volcanic glass) as well, but even through the Neolithic Age, most stone tools were made from chert. Its use in our modern world is nearly as prevalent. Although glass is one of its most familiar products, our society has a remarkable range of uses for quartz.

Large amounts are used as flux in metallurgy and as an abrasive and filler in refractories. Crucibles designed for use at high temperatures are made of fused silica, and pure quartz sands are also used in the manufacture of glass and glass ceramics. Quartz-rich rocks like quartzite and quartz sandstone are often used as building stones and some colored quartz varieties are used as ornamental and semi-precious stones. By volume though, the bulk of all commercially mined quartz is used in the construction industry as aggregate for concrete and as sand in mortar and cement. As a result, from buildings to roads a remarkable amount of our modern infrastructure is built of quartz. Because of its physical strength, ground quartz is used as an abrasive in stonecutting, sandblasting, and scouring soaps. Since it is chemically stable, crushed quartzite fragments are often used as ballast (finely broken rock fragments) along railroad lines and highway shoulders. Pure, fine quartz sands are also used in water purification systems as a filter since it will not react with the water and the pores between the sand grains are small enough to filter out many impurities. Many species of marine plankton, called diatoms, construct their shells of quartz.

As these microscopic organisms die, they sink down to cover the deep sea floor with layers of porous diatom shells. Ancient deep sea diatom deposits now exposed at the Earth’s surface are called diatomaceous earth. Diatomaceous earth has some unusual uses. It is used to filter some of the best wines and as the abrasive part of toothpaste. Since it is composed of quartz, the shells are harder than the apatite and calcite minerals that make up our teeth. When you polish your teeth, you actually are grinding the surface down with a very fine abrasive, often the shells of long dead plankton. One of the more interesting properties of some pure quartz crystals is that they are piezoelectric – that means that when it is put under pressure, the crystal produces an electric voltage. This characteristic allows quartz crystals to be used to measure pressures or control the frequency of electric impulses, which led to their use in radio systems and timepieces. Quartz veins are also important host rocks for gold and other precious metals, and were the target of many mining ‘rushes’. From a historic perspective, however, the single most important use of quartz may have been to start fires.

When chert is struck against iron, it produces a relatively long-lived spark. From prehistoric times on, this has been one of the most reliable and widespread means of starting a fire until the advent of matches, and was the basic firing mechanism of colonial age flintlock muskets. Since quartz is hard and comes in many color varieties, the chert forms of quartz are often used a gemstones. Jasper, tiger’s eye, amethyst, and citrine are all varieties of chert used as gems for jewelry. Onyx is a name used for both a banded variety of quartz and a banded variety of calcite, so don’t be confused by the term’s double use. Both types of onyx are used as decorative stone, but differ in their uses. Calcite onyx is much softer than quartz onyx and is more easily carved, so it is used for most ‘onyx’ carvings. Since it is harder, quartz onyx is more frequently used for jewelry and other objects that must withstand greater handling and wear. Opal is a well-known gemstone often mistaken as a variety of finely crystalline quartz (chert).

Like quartz, it is composed largely of SiO2, but with the addition of water [SiO2-(H20)n]. Technically opal is not a true mineral though, but something called a mineraloid. Mineraloids are amorphous solids, which means that they do not have a set crystal structure. To be a true mineral, a solid must have a crystalline structure. Opal is found as a vein-filling or void-filling material in rocks through which silica-rich waters have moved. There are other minerals, besides quartz, that are also composed solely of silicon dioxide (SiO2), but most of these are only stable at high pressure or temperature, so they rarely occur at the Earth’s surface. Although opal is technically neither a variety of quartz nor even a true mineral, in the jewelry trade it is often considered as both. ^ Top In Our Future: The Environmental Implications of Quartz Use Although quartz is chemically stable and is non-toxic, like any fine particle quartz dust can be hazardous if it is inhaled. Construction workers using sandblasting equipment or even home repair enthusiasts working with cement dust should use good quality masks to avoid breathing airborne quartz dust.

Quartz in the Upper Midwest: Quartz is a very common mineral that can be collected in half the counties in Minnesota and Wisconsin. The best-known occurrences of macroscopic quartz are the Paleozoic sandstone units that mantle much of the region. One of the purest quartz sands in the world is the St. Peter Sandstone that covers parts of southeastern Minnesota and southern Wisconsin. Among its many uses, St. Peter Sandstone outcrops in the Twin Cities area were once the source for most of the automobile glass used in Ford Motor Company cars. Lake Superior agate is probably the most famous regional occurrence of chert (quartz composed of microscopic crystals). These agate masses, originated, as silica was precipitated from fluids to fill pore spaces in ancient volcanic lava flows along the northern border between Minnesota and Wisconsin. As the lava rock weathered, the more chemically resistant quartz fillings were freed to roll around on the gravel beaches of Lake Superior as agate.

Visionlearning | Earth Science | The Silicate Minerals The Silicate Minerals cleavage : breakage in crystal structure of certain minerals along planes where atomic bonds are weakest crust : the outermost layer of Earth; the surface layer of a planet tetrahedron : a figure with four triangular planes; a triangular pyramid Glossary Terms The mineral quartz (SiO2) is found in all rock types and in all parts of the world. It occurs as sand grains in sedimentary rocks, as crystals in both igneous and metamorphic rocks, and in veins that cut through all rock types, sometimes bearing gold or other precious metals. It is so common on Earth's surface that until the late 1700s it was referred to simply as "rock crystal." Today, quartz is what most people picture when they think of the word "crystal." Quartz falls into a group of minerals called the silicates, all of which contain the elements silicon and oxygen in some proportion. Silicates are by far the most common minerals in Earth's crust and mantle , making up 95% of the crust and 97% of the mantle by most estimates. Silicates have a wide variety of physical properties, despite the fact that they often have very similar chemical formulas .

At first glance, for example, the formulas for quartz (SiO2) and olivine ((Fe,Mg)2SiO4) appear fairly similar; these seemingly minor differences, however, reflect very different underlying crystal structures and, therefore, very different physical properties. Among other differences, quartz melts at about 600° C while olivine remains solid to temperatures of nearly twice that; quartz is generally clear and colorless, whereas olivine received its name from its olive green color. The variety and abundance of the silicate minerals is a result of the nature of the silicon atom , and even more specifically, the versatility and stability of silicon when it bonds with oxygen. In fact, pure silicon was not isolated until 1822, when the Swedish chemist Jöns Jakob Berzelius (see the Biography link in our Resources section) finally succeeded in separating silicon from its most common compound , the silicate anion (SiO4)4-. This anion takes the shape of a tetrahedron , with an Si4+ ion at the center and four O2- ions at the corners (see Figure 1); thus, the molecular anion has a net charge of -4.

Figure 1: Three ways of drawing the silica tetrahedron: a) At left, a ball & stick model, showing the silicon cation in orange surrounded by 4 oxygen anions in blue; b) At center, a space filling model; c) At right, a geometric shorthand. The Si-O bonds within this tetrahedral structure are partially ionic and partially covalent , and they are very strong. Silica tetrahedra bond with each other and with a variety of cations in many different ways to form the silicate minerals . Despite the fact that there are many hundreds of silicate minerals, only about 25 are truly common. Therefore, by understanding how these silica tetrahedra form minerals, you will be able to name and identify 95% of the rocks you encounter on Earth's surface . Seeing the structure of the silicates Early mineralogists grouped minerals according to physical properties, which spread the silicates across many groups because they have very different properties.

By the early 1800s, however, Berzelius had begun classifying minerals based on their chemical composition rather than on their physical properties, defining groups such as the oxides and sulfides – and, of course, the silicates. At the time, Berzelius was able to determine the absolute proportions of elements within a mineral, but he could not see the internal arrangement of the atoms of those elements in their crystalline structure. A detailed view of the internal arrangement of atoms within minerals would have to wait over 100 years for the development of X-ray diffraction (XRD) by Max von Laue, and its application to determine atomic distances by the father-son team of William Henry Bragg and William Lawrence Bragg a few years later (see their biographies in our Resources section). In the process of XRD, X-rays are aimed at a crystal . Electrons in the atoms within the crystal interact with the X-rays and cause them to undergo diffraction. In the same way that light can be diffracted by a grate or card (see our Light I: Particle or Wave?

module for more information on this topic), X-rays are diffracted by the crystal and a 2-dimensional pattern of constructive and destructive interference bands results. This pattern can be used to determine the distance between atoms within the crystal structure according to Bragg's Law . The Braggs' work opened up a new world of mineralogy, and they were awarded a Nobel Prize in 1915 for their work determining the crystal structures of NaCl, ZnS, and diamond. XRD revealed that even minerals with similar chemical formulas could have very different crystal structures, strongly influencing those minerals' chemical and physical properties. As scientists created XRD images of the atomic structure of minerals, they were better able to understand the nature of the bonds between atoms in the silicate and other crystals. Within a silica tetrahedron , any single Si-O bond requires half of the available bonding electrons of the O2- ion, meaning that each O2- may bond with a second ion, including another Si4+ ion. The result of this is that the silica tetrahedra can polymerize, or form chain-like compounds , by sharing an oxygen atom with a neighboring silica tetrahedron.

The silicates are, in fact, subdivided based on the shape and bonding pattern of these polymers , because the shape influences the external crystal form, the hardness and cleavage of the mineral, the melting temperature, and the resistance to weathering . These different atomic structures produce recognizable and consistent physical properties, so it is useful to understand the structures at an atomic level in order to identify and classify the silicate minerals. Identifying minerals in a rock may seem like an arcane exercise, but it is only by identifying minerals that we begin to understand the history of a given rock. The most common silicate minerals fall into four types of structures, described in more detail below: isolated tetrahedra, chains of silica tetrahedra, sheets of tetrahedra, and a framework of interconnected tetrahedra. The link below opens a page in a new window, which contains 3-dimensional versions of these different structures. You can manipulate and compare the structures as you read about them. Comprehension Checkpoint The silica tetrahedron is made up of a.a silicon atom surrounded by four oxygen atoms. b.an oxygen atom surrounded by four silicon atoms.

Isolated tetrahedra: Olivine The simplest atomic structure involves individual silica anions and metal cations , usually iron (Fe) and magnesium (Mg), both of which exist most commonly as ions with charge of +2. Therefore, it takes two atoms of Fe2+ or Mg2+ (or one of each) to balance the -4 charge of the silica anion. Olivine (see Figures 2a and 2b below) is the most common silicate of this type, and it makes up most of the mantle . Because these minerals contain a relatively high proportion of iron and magnesium, they tend to be both dense and dark-colored. Because the tetrahedra are not polymerized, there are no consistent planes of internal atomic weakness, so they also have no cleavage . Garnet is another common mineral with this structure. Figure 2a: Depiction of a single silicate tetrahedron. Figure 2b: A picture of olivine (the green crystals), an example of a silicate structure composed of isolated tetrahedrons, with a vein of basalt (the gray material).

Chains of tetrahedra: Pyroxenes and amphiboles When silicate anions polymerize, they share an oxygen atom with a neighboring tetrahedron . Commonly, each tetrahedron will share two of its oxygen atoms, forming long chain structures. These chains still have a net negative charge , however, and the chains bond to metal cations like Fe2+, Mg2+, and Ca2+ to balance the negative charge. These metal cations commonly bond to multiple chains, forming bridges between the chains. Single-chain silicates include a common group called the pyroxenes, which are generally dark-colored (see Figures 3a and 3b). Because the bonds within the tetrahedra are strong, planes of atomic weakness do not cross the chains; instead, pyroxenes have two cleavage planes parallel to the chains and at nearly right angles to each other. Figure 3a: A schematic diagram of the single chain silica structure. Where two tetrahedra touch, they share an oxygen ion. Figure 3b: Pyroxene is one of the dominant minerals in this sample of gabbro. It is the dark mineral and can be hard to recognize.

Double chains form when every other tetrahedron in a single chain shares a third oxygen ion with an adjoining chain (see Figure 4a). Like single chains, the double chains still maintain a net negative charge and bond to cations that can form bridges between multiple double chains. Figure 4a: A schematic diagram of the double chain silicate structure. Double chain silicates, called amphiboles, host a wider variety of cations , including Fe2+, Mg2+, Ca2+, Al3+, and Na+, and have a wide variety of colors. The most common amphibole is hornblende, a black mineral found in igneous rocks like granite and andesite (see Figures 4b and 4c). Amphiboles tend to form prismatic crystals with two cleavage planes at 120 degrees to each other. Figure 4b: Individual hornblende crystals where the characteristic cleavage can be seen. Figure 4c: Hornblende is the dark mineral in this rock. Pyroxenes and amphiboles can be difficult to distinguish from one another, as they are both dark-colored, blocky minerals . A careful examination of the angle between cleavage planes, described above, is required to identify them.

Comprehension Checkpoint The best way to tell single-chain silicate minerals from double-chain silicate minerals is by examining their a.color. Quartz has no cleavage because a.its framework structure has no natural weaknesses. b.it is identical in structure to feldspar. Silicates as a natural resource Though we generally think of coal or oil when discussing natural resources , silicate minerals are a natural resource we can't live without on our planet, and not just because of our increasing reliance on computers. Without quartz , there would be no glass. Without the clay minerals, we would have no ceramics or pottery. We use silicate minerals in the manufacture of many building materials, including bricks and concrete. The weathering of silicate minerals on the surface of Earth produces the soils in which we grow our foods and the sand on our beaches. The properties of the minerals that are important to us are based on the versatility of the silicate anion in combination with other elements . Summary Understanding the structure of silicate minerals makes it possible to identify 95% of the rocks on Earth. This module covers the structure of silicates, the most common minerals in the Earth's crust.

The module explains the significance of the silica tetrahedron and describes the variety of shapes it takes. X-ray diffraction is discussed in relation to understanding the atomic structure of minerals. Key Concepts Silicate minerals are the most common of Earth's minerals and include quartz, feldspar, mica, amphibole, pyroxene, and olivine. Silica tetrahedra, made up of silicon and oxygen, form chains, sheets, and frameworks, and bond with other cations to form silicate minerals. X-ray diffraction (XRD) allows scientists to determine the crystal structure of minerals. The physical properties of silicate minerals are determined largely by the crystal structure. NGSS

Question: Of which common mineral is rock crystal a variety
Answer: Quartz